BBC - h2g2 - Advanced Electronic Countermeasures

The History of Radar | Radar History: Isle of Wight Radar During The Second World War | Radar: The Basic Principle
Radar Technology: Main Components | Radar Technology: Side Lobe Suppression | Radar Technology: Airborne Collision Avoidance
Radar Technology: Antennas | Radar Technology: Antenna Beam Shapes | Radar Technology: Monopulse Antennas | Radar Technology: Phased Array Antennas | Radar Technology: Continuous Wave Radar | Theoretical Basics: The Radar Equation
Theoretical Basics: Ambiguous Measurements | Theoretical Basics: Signals and Range Resolution
Theoretical Basics: Ambiguity And The Influence of PRFs | Theoretical Basics: Signal Processing | Civilian Radars: Police Radar | Civilian Radars: Automotive Radar | Civilian Radars: Primary and Secondary Radar
Civilian Radars: Synthetic Aperture Radar (SAR) | Military Applications: Overview | Military Radars: Over The Horizon (OTH) Radar
How a Bat's Sensor Works | Low Probability of Intercept (LPI) Radar | Electronic Combat: Overview | Electronic Combat in Wildlife
Radar Countermeasures: Range Gate Pull-Off | Radar Countermeasures: Inverse Gain Jamming | Advanced Electronic Countermeasures

With all its powers of signal processing and EPM features like frequency-hopping and adaptive beam-shaping, a modern air defence radar is hard to overcome. ECM techniques like range gate pull-off may or may not work as desired. If nothing helps an aircraft from being detected and tracked and a missile is on its way then the last resort for an attacking aircraft is to provide something else as a target and to lure the missile seeker away from itself. This 'something else' can be a real thing, or it can be... well, read on!

Missile Seeker Heads

The seeker head of a radar-guided missile consists of a radar receiver, an antenna with some direction finding capability and logic circuitry which guides the missile into the direction of an incoming radar signal. This signal can be provided by a transmitter carried in the missile itself (the seeker-head is called 'active'), a ground-based target illumination radar (the seeker-head is called 'passive'), or the target in question (that is, the missile homes in on the aircraft's own radar or on its on-board jammer). In any case, the circuitry in the seeker head assumes that the origin of the signal in its receiving path is identical with the target itself.

It is the task of advanced ECM to make this assumption wrong.

Towed Decoys

Towed decoys are radar transmitters which operate as signal repeaters and are literally towed behind an aircraft by means of a cable. The cable includes a power supply line and conveys the signals which are generated by the radar-warning receiver (RWR) and control the transmitter's operation.

The RWR features a 'signal library,' a database which is derived from ELINT findings. If signals are detected which are known to belong to an anti-aircraft missile then the RWR commands the transmitter in the towed decoy to either broadcast an amplified copy of the impinging signal, or simply noise. In the first case, the false echo transmitted from the decoy is much stronger than the 'skin' echo returned from the aircraft's fuselage, and the seeker-head will (hopefully) switch over to homing in on the decoy. In the second case, the seeker electronics experience an unability to measure range, therefore they switch over into a 'home-on-jam' mode of operation (which essentially means to aim at the source of jamming) and proceed to home in on the decoy.

Therefore, the missile has been lured away from its real target. The decoy very small and so stands a chance of not getting hit at all, and even if it is destroyed then at least the aircraft will survive the attack.

Chaff Illumination

Chaff denotes small strips of metal (or glass fibres coated with metal) which are cut so that their length equals half of the wavelength of a radar's signal. This makes each piece a dipole antenna with quite impressive reflecting properties. As the history of radar has shown, chaff is quite successful against an unsophisticated radar but nowadays is rather limited in its efficiency if applied in its pure form.

But things are different if an aircraft under attack launches a chaff cloud and an on-board rearward-pointing antenna starts re-transmitting an incoming radar signal at it. That is, the chaff cloud is used as a mirror (a very blurred mirror, but a mirror), and the missile-seeker is presented with a strong false target and will switch over to homing in on the brightly-lit cloud of chaff.

Terrain Bounce

As can be seen from watching a sunset on a beach, the flat surface of water is quite good a reflector of light. The same holds true for electromagnetic waves in general. Any surface can be considered 'flat' with respect to electromagnetic waves, as long as irregularities remain short in comparison to the wavelength. Thus, soil which is moist from rain or dew, like a wet golf course, makes up a radar mirror too.

If carried on low-flying aircraft, terrain bounce jammers can make use of this effect by picking up a missile-seeker's signal, amplifying it and re-transmitting it down to the ground. The seeker-head is presented with the choice between a weak skin echo and a bright mirror image and hopefully will switch over to homing in on the spot on the ground.

Conclusion

In electronic combat, there are more things in the sky and on earth than are dreamt of by philosophy. And, as in a maze of mirrors, you can never trust your eyes.